1. Field of Invention
The present invention relates to a technology for a system in which an adsorbent is provided in an exhaust passage of an internal combustion engine for adsorbing unburned fuel components in exhaust gas, the technology being capable of determining an amount of unburned fuel components adsorbed to the adsorbent.
2. Description of Related Art
A typical internal combustion engine installed in a motor vehicle or the like is equipped with an emission control catalyst device provided in a pathway of an exhaust passage for substantially eliminating or lessening harmful gas components present in the exhaust gas. An example of such an emission control catalyst device is a three-way catalyst device in which a surface of a ceramic support is coated with alumina, and the alumina surface is loaded with a platinum-rhodium-based precious-metal catalytic substance. This type of three-way catalyst device causes hydrocarbons (HC) and carbon monoxide (CO) contained in exhaust gas to react with oxygen (O.sub.2) present in exhaust gas, thereby oxidizing them into water (H.sub.2 O) and carbon dioxide (CO.sub.2), when the air-fuel ratio of exhaust gas flowing into the catalyst device is at or close to the theoretical air-fuel ratio. Simultaneously, the three-way catalyst device causes oxides of nitrogen (NOx) present in exhaust gas to react with hydrocarbons (HC) and carbon monoxide (CO) present in exhaust gas, thereby reducing NOx into water (H.sub.2 O), carbon dioxide (CO.sub.2) and nitrogen (N.sub.2).
The above-described three-way catalyst device is able to substantially eliminate or lessen unburned hydrocarbons, carbon monoxide, and oxides of nitrogen contained in exhaust gas, thereby preventing those harmful gas components from being released into the atmosphere.
Normally, the three-way catalyst device activates at or above a predetermined activation temperature (e.g., 800.degree. C. to 500.degree. C.) so as to significantly lessen harmful gas components in exhaust gas. That is, when the temperature of the catalyst device is below the activation temperature, the catalyst device is inactive and cannot significantly lessen harmful gas components in the exhaust gas.
Particularly, at the time of a cold start of the internal combustion engine, combustion in the engine becomes unstable while the amount of fuel injected is increased from a normal level in order to facilitate the start of the engine and to accelerate warm-up. Therefore, the amount of unburned fuel components (e.g., hydrocarbons) contained in exhaust gas increase in some cases. If the three-way catalyst device is not activated in such a case, there is a possibility that unburned hydrocarbons will not be significantly lessened, but will be released in large amounts into the atmosphere.
To overcome this problem, a technology has been proposed in which an exhaust passage is provided with an HC adsorbent that is formed from a porous zeolite material or the like and that adsorbs unburned hydrocarbons in exhaust gas when a predetermined temperature is not reached, and that desorbs unburned hydrocarbons (previously adsorbed) when the predetermined temperature is reached or exceeded. When a three-way catalyst device is inactive, unburned hydrocarbons in exhaust gas are caused to adsorb to the HC adsorbent so as to prevent release of unburned hydrocarbons into the atmosphere.
If an internal combustion engine is equipped with an HC adsorbent as described above, it may be conceivable to estimate an amount of unburned hydrocarbons adsorbed to the HC adsorbent and, on the basis of the estimated value, determine whether the HC adsorbent has deteriorated. To realize this goal, it is important to accurately estimate an amount of unburned hydrocarbons adsorbed to the HC adsorbent.
As a technology that addresses such a requirement, an internal combustion engine emission control apparatus is described in Japanese Patent Application Laid-Open No. HEI 6-93846. The emission control apparatus detects the amount of hydrocarbons discharged from the engine, the flow rate of exhaust gas passing through an HC adsorbent, and the temperature of the HC adsorbent. Based on the detected values, the apparatus estimates a total amount of hydrocarbons adsorbed to the HC adsorbent.
In the apparatus described in JP 6-93846, the amount of unburned hydrocarbons discharged from the internal combustion engine is calculated on the basis of the amount of fuel injected, the degree of fuel vaporization (which is estimated from the temperature of engine-cooling water), the flow of exhaust gas (which is estimated from the engine revolution speed), and the like. Subsequently, a rate of adsorption to the HC adsorbent is estimated from the flow rate of exhaust gas passing through the HC adsorbent and the temperature of the HC adsorbent. Based on the rate of adsorption to the HC adsorbent and the amount of unburned hydrocarbons discharged from the internal combustion engine, an amount of unburned hydrocarbons expected to be adsorbed by the HC adsorbent is estimated.
Under a condition where unburned hydrocarbons in exhaust gas are caused to adsorb to the adsorbent, passage portions that are located upstream of the adsorbent and that conduct fuel components, such as an intake passage, a combustion chamber, an exhaust passage and the like, have relatively low temperatures, so that those passage portions are likely to receive adherents in the fuel components. Therefore, the amount of unburned fuel components actually adsorbed to the adsorbent varies in accordance with the amounts of fuel components adhered to wall surfaces or the like of the passage portions.
Particularly at the time of a cold start of an internal combustion engine, the temperatures of the intake passage, the combustion chamber, the exhaust passage and the like are relatively low, so that fuel components do not readily vaporize but are likely to adhere to wall surfaces of the passages. If the amount of such unburned hydrocarbon adherents is not taken into account, it becomes difficult to precisely determine an amount of unburned hydrocarbons discharged from the engine, so that it becomes difficult to determine, with high precision, the amount of unburned hydrocarbons adsorbed to the HC adsorbent.
Furthermore, if an emission control catalyst device is provided in a portion of the exhaust passage upstream of the HC adsorbent, portions of unburned hydrocarbons present in exhaust gas temporarily adsorb or adhere to the emission control catalyst device. Therefore, the amount of unburned hydrocarbons temporarily adsorbed or adhered as described above also need to be taken into account, in order to accurately estimate an amount of unburned hydrocarbons adsorbed to the HC adsorbent.
Still further, the amount of unburned hydrocarbons adsorbed to the HC adsorbent changes in accordance with degrees of performance deterioration of the emission control catalyst device. Therefore, it is necessary to consider the performance deterioration of the emission control catalyst device as well.